Among various optical pickups of different sorts, there is such a one which is capable of recording and/or reproducing two sorts of the optical discs of different physical formats, such as a Compact Disc (CD) and a Digital Versatile Disc (DVD). An optical pickup capable of recording and/or reproducing these two sorts of the optical discs is provided with a light source for radiating a light beam for CD, having a wavelength of 780 nm, and a light beam for DVD, having a wavelength of 635 to 650 nm, a double focal point type objective lens for condensing the light beam radiated from the light source, a photodetector for detecting the return light beam reflected back from the signal recording surface of the CD or the DVD, and an objective lens driving unit for driving the objective lens into displacement in the focussing and tracking directions.
The objective lens driving unit is provided with a lens holder for holding the objective lens, a holder supporting member for supporting the lens holder by a resilient supporting member for displacement in the focussing and tracking directions, a focussing driving unit for actuating the lens holder holding the objective lens in the focussing direction, and a tracking driving unit for actuating the lens holder holding the objective lens in the tracking direction.
The focussing driving unit is provided with a focussing coil and a focussing magnet, and actuates the lens holder, held by the resilient supporting member, into displacement along the focussing direction, which is the direction of the optical axis of the objective lens, under the action of the current flowing through the focussing coil and the magnetic field generated by the focussing magnet. The tracking driving unit is made up by a tracking coil and a tracking magnet, and actuates the lens holder, carried by the resilient supporting member, into displacement along the direction perpendicular to the optical axis of the objective lens, under the action of the current flowing through the tracking coil and the magnetic field generated by the tracking magnet.
With the above-described optical pickup, the objective lens is actuated by the focussing driving unit into displacement for focussing the light beam radiated from the light source on the signal recording surface of the optical disc. The objective lens is actuated by the focussing driving unit into displacement for focussing the light beam radiated from the light source on the signal recording surface of the optical disc. By actuating the objective lens by the tracking driving unit into displacement in the tracking direction, the light beam is scanned to read out information signals recorded on the CD or on the DVD.
The optical pickup is further mounted on a base unit adapted to be mounted to the recording and/or reproducing apparatus. This base unit includes a base mounted to a casing of a recording and/or reproducing apparatus. The base, in turn, includes a slide member, on which the optical pickup is mounted, a feed unit for advancing the slide member, on which the optical pickup is mounted, along the radius of the optical disc, and a disc rotating actuating unit for rotationally driving the optical disc.
Meanwhile, the optical pickup, assembled in the assembling step, is adjusted as to the relative positions between the objective lens and the light source and as to tilt of the optical axis of the objective lens. This adjustment is carried out after mounting the optical pickup, such as after mounting the optical pickup to the base unit. In this case, the relative positions between the objective lens and the light source and the tilt of the optical axis of the objective lens can be adjusted without dependency on the mounting precision of the optical pickup to the base unit, in contradistinction from the case of carrying out the adjustment before mounting the optical pickup to the base unit.
This adjustment is carried out for each of the CD and the DVD. Specifically, the optical disc for CD adjustment is loaded on the disc rotating actuating unit and run in rotation for adjusting the relative positions between the objective lens and the light source for optimizing the optical characteristics. The optical disc for DVD adjustment then is mounted on the disc rotating actuating unit and is run in rotation for adjusting the relative positions between the objective lens and the light source for optimizing the optical characteristics. Subsequently, the tilt of the objective lens is adjusted, using the optical disc for DVD adjustment.
It should be noted that the optical disc for CD adjustment has 8-14 modulated data spirally recorded thereon, in accordance with the same physical format as that for the CD, while the optical disc for DVD adjustment has 8-16 modulated data spirally recorded thereon in accordance with the same physical format as that for the DVD, that is with a physical format for recording to a higher density than with the physical format for the CD.
In making adjustments of the optical pickup used for recording and/or reproduction of two optical discs, that is CD and DVD, it is necessary to get the optical pickup for CD adjustment mounted on the disc rotating actuating unit, when adjusting the optical characteristics for reproducing the CD, while it is necessary to get the optical pickup for DVD adjustment mounted on the disc rotating actuating unit, when adjusting the optical characteristics for reproducing the DVD. In making these adjustments, the operations of exchanging the optical disc for adjustment is necessitated, with the consequence that it is difficult to attempt to improve the efficiency of the optical pickup adjustment operation further.
Moreover, these adjustments are carried out as the light source or the optical system is held by the light source adjustment unit of the adjustment apparatus in such a state that the slide member for advancing the optical pickup and the lens holder for holding the objective lens or the holder supporting member are held to high accuracy At this time, the slide member, holder supporting member and the light source are held separately, and are moved relative to one another in only a minor quantity for making the adjustments, so that, if the optical pickup in its entirety is advanced across the inner and outer rims of the optical disc for adjustment, difficulties are encountered in making the above adjustments.
In any of the above-described optical discs for adjustment, data are recorded spirally. Thus, if, as the optical disc for adjustment is kept in rotation, data is to be read out from the recording track of the optical disc for adjustment, the objective lens is moved gradually towards the outer rim of the disc simultaneously with rotation of the optical disc for adjustment. Thus, during adjustment, the objective lens of the optical pickup is moved towards the outer rim of the optical disc for adjustment, with the objective lens being moved from the zero viewing field swing state to the viewing field swing state, with the consequence that the optical axis of the objective lens is offset relative to the optical design center, such as the center of the light source, referred to below as the optical center. With this adjustment method, offsetting of the optical axis of the objective lens from the optical center leads to deteriorated optical characteristics and to deteriorated jitter of detected replay signals. Thus, in case changes in the reproduction signals are measured by tilting the optical axis of the objective lens to measure changes in the reproduced signals to adjust the tilt of the optical axis of the objective lens to an optimum point, considerable difficulties are encountered in making such adjustments.
For overcoming this deficiency, there is known such a method in which, when the optical axis of the objective lens has become offset a preset amount relative to the optical center, the feed unit is actuated to disengage the tracking servo to perform feed movement of the objective lens by a distance corresponding to the offset towards the inner rim of the optical disc to get the optical axis of the objective lens located within a predetermined offset at all times relative to the optical center. This feed movement is termed below a track jump.
In this method, since the optical axis of the objective lens and the optical center are moved at all times, it is difficult to detect real values during adjustment. Moreover, in this method, the time until tracking servo is in operation is short, while it takes time until stable real values are measured by a measurement unit, such as a jitter detector. Consequently, the time within which the position of the optical axis of the objective lens can be adjusted after track jump becomes very short, with the consequence that considerable difficulties are again encountered in making the adjustments Moreover, if, with the present method, the track jump interval is increased, the position offset of the optical axis of the objective lens is increased further.
In addition, since the data read out from the optical disc for CD adjustment is 8-14 modulated data, while the data read out from the optical disc for DVD adjustment is 8-16 modulated data, the adjustment apparatus needs to be provided with a signal processing circuit including the two modulating circuits, thus complicating the structure of the reproduction system.